Tacrolimus is a macrolide lactone also known as FK506, fugimycin or tsukuba-enolide, which is a pharmaceutically active compound, i.e. a drug substance. A number of tacrolimus formulations are marketed, e.g. under the tradenames Prograf®, Advagraf®, and Protopic®, and used as immunosuppressive agents to prevent allograft rejection, i.e. rejection of transplanted organs. Tacrolimus formulations may also be used topically in a number of conditions.
Tacrolimus is produced by the bacterium Streptomyces tsukubaensis and the name is an acronym reportedly derived from“tsukuba macrolide immunosuppressant”. Tacrolimus belongs to the class of L-pipecolic acid derived macrolides produced by Streptomyces species also comprising for example rapamycin (sirolimus), ascomycin and meridamycin, which may have valuable pharmacological properties.

In 2009, the USP issued a draft monograph for tacrolimus monohydrate (Pharmacopeial Forum 35(2) [March-April 2009], pp. 310-314, USP Pharmacopeial Convention Inc.) using a systematic name for tacrolimus based not on the IUPAC/octacosa-structure convention but on the natural products convention, wherein the base structure is taken to be tricosine (a three-membered macrocycle). This is different from the IUPAC-style nomenclature used in U.S. Pat. No. 4,894,366 for the tacrolimus structure based on a 28-membered ring (octacosa-) numbered clockwise from a carbon in the tetrahydropyran ring.
Tacrolimus exerts its action by binding to the immunophilin FKBP12 to give a complex that inhibits calcineurin, a calcium-dependent phosphatase participating in signal transduction that leads to lymphokine gene transcription.
The presence of degradation products in a pharmaceutical formulation, including a pharmaceutical composition comprising tacrolimus as an active pharmaceutical ingredient, is highly undesirable, since it imposes an increased risk to the patients.
Further, very strict regulatory restrictions exist regarding impurities present in a pharmaceutical formulation, both in a newly prepared pharmaceutical formulation, and in pharmaceutical formulations upon storage, i.e. during their shelf-life. Accordingly, it is necessary to monitor and document the formation of any possible degradation product stemming from the pharmaceutical formulation itself, notably any degradation product arising from the active ingredient, and to either control the amount of degradation product present in the formulation during shelf-life, or to prevent or reduce the formation of degradation product during manufacturing or the shelf-life of the formulation, depending on the nature of each degradation product that may be present or in the pharmaceutical formulation product.
Tacrolimus is a poorly soluble substance, which when administered in crystalline form, is likely to have very low bioavailability due to the relative low amount of liquid available for solubilization in the gastrointestinal tract, especially in the lower part of the intestines including the colon. Accordingly, several attempts have been made to prepare solid solutions, preferably in the form of solid dispersions as also disclosed early in the development of tacrolimus formulations by Hone et al, Transplantation Proceedings, Vol XIX, No 5, Suppl 6 (October), 1987: pp 17-22, which discloses solid dispersions with different formulations. “Establishment of new preparation method for solid dispersion formulation of tacrolimus” by YAMASHITA Kazunari et al, International Journal of Pharmaceutics 2003, vol. 267, no1-2, pp. 79-91 discloses an improved solvent method in order to prevent the use of dichloromethane.
Solvents are generally undesirable in the manufacturing of pharmaceuticals, potential trace amounts need close monitoring, and manufacturing involving solvents is costly. Thus, it is highly desirable for many pharmaceutical companies to be able to prepare solid dispersions without the use of solvents, so that the drug market can be entered with approved products having sufficient absorption capability despite low solubility of the active ingredient. However, the downside of such solid dispersions, especially molecular dispersions, is the motility and the increased exposure of the molecules in the formulation, which increases the risk of chemical degradation compared with conventional crystalline formulations. Identification and prevention of degradation products counts for a considerable part of the cost during development of new pharmaceutical compositions or formulations, and, eventually, otherwise improved formulations may fail to reach the market, if the degradation cannot be controlled in the preferred vehicles of the molecular dispersion of the active ingredient, i.e. the drug substance.
WO2005/020993, WO2005/020994, WO2008/0145143 and WO2010/005980 disclose tacrolimus-containing pharmaceutical compositions with improved bioavailability and a reduced peak-to-trough level as compared to the commercially available tacrolimus products, in particular tacrolimus compositions comprising a solid dispersion of tacrolimus in polyethylene glycol (PEG).
For tacrolimus-containing formulations, in particular formulations containing ingredients which may, as starting materials in the manufacturing process, contain traces of metals or metal compounds, oxidants and other undesirable but unavoidable contaminants, there is a need for preventing the formation of degradation products from tacrolimus or, at least, to maintain an acceptable, low concentration of such degradation products throughout the shelf-life of the formulation, which typically is a formulation in a unit dosage form such as a capsule (soft or hard), a tablet, or granules in a sachet, or as an injection liquid, or as a topical product.